DE2414478B2 - AEROGELY STRUCTURED SILICA AND METHOD FOR MANUFACTURING IT - Google Patents

Description

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The invention relates to airgel-like structured wi Silicic acid and a process for its preparation.

Aerogels include silica gels with a low bulk density (approx. 20 to 50 g / l) and high macroporosity (DBP numbers up to 3.4 ml / g). Due to the contraction effect described by R. 11 e r, that when drying silica gels from the aqueous phase leads to a breakdown of the pore structure leads, silica aerogels are only after that described by K is tier in US Pat. No. 2,249,767 Procedure accessible. Here, K is tier partially dehydrates silica sols with alcohols and then dries them then the water-containing alcoge! with an autoclave by releasing the pressure under supercritical conditions. Au In this way, he receives particularly loosely structured structures from the primary particles of the silica brine the high volume values in the area of macropores (> 300 Å) with a very low apparent density (rubble weight) have.

These substances, known as airgel, are used depending on their degree of porosity and bulk density Fillers, carrier silicas, matting agents, thickening agents, etc. The method of manufacture of these aerogels is due to the necessary use of organic solvents and Supercritical drying to be carried out is technically and economically very complex.

It has now been found that so far, the avoid for preparing the AerogeJe zui available methods portrayed the disadvantages, if, instead of using the silica sol particles of a primary particle fumed silicas and dispense with a recovery from the liquid phase.

The primary particles of pyrogenic silica lie loosely associated with electrostatic beds in bulk and van der Waals forces. They form flakes, which have a high air content and therefore have a very high apparent pore volume and a correspondingly low bulk density. A pour these flakes are best referred to as air dispersion. The flakes, however, are unstable and In contrast to the secondary particles of Kistler's aerogels, they become more mechanical at the slightest Stress reduced down to the primary particles.

DT-PS 10 75 573 describes the hydrophilization and deacidification of fluorine-containing silicon dioxide by treatment with steam at temperatures of about 100 to about 1000 ^° C. in order to avoid particle enlargement even after long storage (column 1, lines 9, 10).

The invention relates to an airgel-like structured silica with a BET surface area according to DIN 66 131 between 80 and 450m ² / g, a bulk density between 10 and 60 g / l of a DBP number - the determination of the DBP number (dibutyl phthalate adsorption) is carried out according to DT-PS 17 67 332, column 2, lines 30-64 - between 2.4 and 3.8, a mean particle size range of 0.1 to 7 μm, a most common particle size range of 1 to 2 μm and a pH Value in 4% aqueous slurry of 6 to 8, obtained by incorporating 5 to 50% by weight of water adjusted to a pH value between 7 and 14 into air-dispersed fumed silica with a bulk density of 10 to 60 g / l and one BET surface area between 100 and 480 m ² / g with uniform distribution and drying of the powdery mixture obtained.

Another object of the invention is a process for the production of airgel-like structured silicas, which is characterized in that air-dispersed pyrogenic silica with a bulk density of 10-60 g / l and a BET surface area between 100 and 480 m ² / g 5 - 50% by weight of water adjusted to a pH value between 7 and 14 is incorporated with uniform distribution and the powdery mixture obtained dries.

As the volume of silica when incorporating of the water decreases only slightly, it can be assumed that the originally existing association of Primary particles of air-dispersed fumed silica remains largely up-to-date. Through the -. Loading with water is likely to result in one Partial dissolution of the silica surface, so that dissolved silica is present here. This cemented the following Dry the primary particles at their points of contact. Hi

It is created through targeted loading with water and subsequent drying from a pyrogenic Silicic acid is a dispersion-stable substance with a high macropore volume corresponding to the Kistler aerogels and very low apparent density r> (peel weight).

It was also found that the apparent, structure determined by the packing density of the fumed silica in air, which by its apparent Density (bulk density) is expressed, a clear influence on the according to the invention Process accessible product: The more voluminous the starting product, the more voluminous it is End product. r>

It has been found to be expedient to use pyrogenic products for the production of the products according to the invention Silicic acid with a bulk density of 15 to 30, in particular around 20 g / l, is used.

In addition, it has proven to be advantageous to choose pyrogenic silica with large surface areas and thus small primary particles. According to a favorable embodiment of the process according to the invention, silica with BET surface areas between 250 and 300 m ² / g is used. s>

Complete wetting of the primary particles can already be achieved if 5 to 20, in particular 7 to 15% by weight, of water are incorporated into the silica with uniform distribution. Since the incorporated water is wegzutrocknen again, the smallest possible amount of water is desirable from economic reasons w chen. However, the amount required depends to some extent on the type of incorporation.

The structure build-up according to the method according to the invention can be promoted noticeably if one -ti the water basically reacting compounds such. B. ammonia, sodium, potassium hydroxide, water-soluble Amines, water glass or the like added. The additional amounts are expediently chosen so that a pH of 8 to 12, in particular 10 to> 11, sets in the water.

The alkalis used act as solubilizers for silica and cause an increase in the Macroporosity of the process products.

Instead of alkaline compounds, free silicic acid or hydrolytic silicic acid and / or alkali-releasing substances can also be added to the v-, water. B. by acidification or ion exchange of silicate solutions or by hydrolytic cleavage of organosilicon compounds. w> z. B. from tetramethylsilicate, generated silica promotes the structure build-up as well. A hydrolytically alkali and silicic acid releasing substance is z. B. sodium nicthyl siliconate.

The uniform distribution of the water in the η ^γ > silica can be carried out by seeding or spraying into the agitated mixing silica at temperatures of the silica between 20 and 100, preferably 40 and 70, in particular 50 and H) C. The mixing movement is expediently carried out by stirring.

Another variant of the introduction of water exists in that you put the water in a, z. B. in a downpipe, fluidize the mass flow of the silica sprayed.

It has also proven advantageous to carry out the water loading at moderately elevated temperatures. This can be done by preheating either the water to be incorporated or the silica or both components. Thus, the water to be incorporated can have a temperature between 20 and 100, preferably have 50 and 100, in particular 90 and 100 ⁰ C.

The structure can also be promoted by briefly dampening the loaded silica in the closed space. Steaming leads to particularly good water distribution. In this has proved to be low, the water-laden silica, before drying, in sealed vessel for about 5 to 60, preferably 10 to 30, in particular 20 minutes, at temperatures up to the boiling point of water, preferably 50 to 80, in particular to 6O ⁰ C to dampen.

Another way to improve the water distribution is that one of the water-laden Silica ground, for example, on pin or air jet mills.

It is then dried, with the preformed structure presumably loosening over the surface or primary particles coated on the surface with free silica is fixed.

The type of drying is not very critical. One can use the prepared mixture of silica and water, which phenomenologically always resembles a dry powder, for example in the tray, plate, Büttner, Dry flow or microwave dryers. The water-laden silica can, however, also under Saving a separate process step in a steam or air jet mill at the same time and be dried.

Provided a separate drying of the powdery mixture obtained after loading with water is carried out, you can connect a dry grinding on a pin or air jet mill.

The invention further relates to the use of the airgel-like structured silica described as a matting agent.

The invention is further explained below by means of exemplary embodiments.

example 1

In a Lödige mixer, pyrogenic silica with a bulk density of 25 g / l and a BET surface area of 340 m ² / g was moistened with 20 ml of water which had a pH of 6.8. The moistening took place at room temperature with mixing stirring. The water was added dropwise from a burette over a period of 18 minutes.

A dry-acting powder with a water content of 15.3% by weight was obtained. This powder was ground to a counter-rotating pin mill and dried in a laboratory drying oven at 120 ⁰ C.

E ;; became a loose, opalescent powder with a bulk density of 17.8 g / l and a DBP number of 2.78 ml / g obtained.

The effectiveness as a matting agent was shown in a poly urethane coating tested. The result is shown in Example 9.

The product obtained and the starting material were suspended in water and ultrasound dispersed.

In each case one drop of this dispersion was dried on an object diaphragm. Electron microscope images of the samples show that the product structured according to the invention in the form of dispersion-stable, discrete secondary particles in the size range predominantly between I and ΙΟμηι is present. In contrast the starting material is practically completely in the form of primary particles.

The following should be noted about the two electron microscope images:

Fig. 1 shows that in a magnification of 1: 5000 Starting material according to Example 1, which is ultrasonically dispersed in water and dried. The primary particles are largely discreet.

Fig. 2 shows that in a magnification of 1: 5000 End product according to Example 1, which is ultrasonically dispersed in water and dried. Training a secondary structure can be clearly seen.

Example 2

In a Lödige mixer, 5 l of a pyrogenic silica with a bulk density of 19.3 g / l and a BET surface area of 189 m ² / g were mixed with 20 ml of water which had been adjusted to a pH of 11.3 with sodium hydroxide solution, moisturizes.

The moistening took place at room temperature with mixing stirring over a period of 20 minutes The water was added dropwise from a burette.

A dry-acting powder with a water content of 16% was obtained.

Since the powder contained tangible solid components, the apparently due to the collapse of the apparent structure of fumed silica by local Was due to excess water, the powder was ground on a pin mill and in one Laboratory drying cabinet dried at 120 ° C.

A loose, opalescent powder with a bulk density of 17.5 g / l, a BET surface area of 153 m ² / g, a DBP number of 2.6 ml / g, a water pore volume according to Innes of 1.9 ml / g and a residual water content of 2.6%.

The matting effect of the substance was made in poly-urethane coating checked and can be found in example 9

Example 3

Fumed silica with a bulk density of · -, ■ -. 27 g / 1 and a BET surface area of 297 mVg.

In this mass flow flowing in the downpipe, 1.5 parts of ammoniacal were added to 10 parts of silica Injected water with a pH value of 10.1. It w> became a dry-acting powder with a bulk density of 26 g / l and a water content of 12% by weight obtained.

This powder was dried in a microwave oven. _h5

An opalescent, loose powder with a bulk density of 23 g / l, a BET surface area of 270 m ² / g and a DBP number of 3.1 ml / g was obtained.

The matting effect of the substance was tested in a poly-urethane coating and is shown in Example 9 / remove.

B e i s ρ i e I 4

Wasserbeladcnes product from Example 3 was ground to improve the water distribution in an air jet mill at 3.1 atmospheres grinding pressure, 3.2 atmospheres injector pressure to a feed rate of 8.8 kg per hour, drying to 3.2 wt .-% residual water ii was a laboratory drying oven at 120 ⁰ C.

A loose, opalescent powder was obtained with a bulk density of 14 g / l, a BET surface area of 277 m ² / g, a DBP number of 3.2 ml / g and an Innes water pore volume of 2.4 ml /G

The matting effect of the substance was tested in a poly-urethane coating and is Example 9 zi remove.

Example 5

Product loaded with 12% by weight of water Example 3 was ground on a counter-rotating pin mill and in a closed, preheated one Powder bottle steamed for 20 min. At 55 ° C and then ßend at 120 ° C in a laboratory drying cabinet on a Residual moisture of 3 wt .-% dried.

A loose, opalescent powder with a bulk density of 16 g / l, a BF.T surface (of 264 m ² / g, a DBP number of 3.78 ml / g and an Innes water pore volume of 2.4 ml / j obtained.

The matting effect of the substance was tested in a poly urethane coating and is shown in Example 9 zi remove.

Example 6

Fumed silica with a bulk density of 54 g / l and a BET surface area of 311 mVg was fed into a heated downpipe heated to 70 ° C. via a metering screw. In this fluidized mass flow, 3 parts of 80 ^° C. warm water, which had been adjusted to a pH of 10.4 with water glass, were injected into 10 parts of silica.

An apparently dry powder with a bulk density of 80 g / l and a water content was obtained of 18.3% by weight. This powder was on the air jet mill mentioned in Example 4 among the there Grind specified conditions and then on a plate dryer with a plate temperature dried from 127 ° C to a residual water content of 4.2%.

An opalescent, loose powder with a bulk density of 36 g / l and a BET surface area was obtained of 260 mVg and a DBP number of 3.2 ml / g.

The matting effect of the substance was tested in a Holy urethane coating and is an example! 9 to remove.

Example 7

Dry product from Example 5 was ground again on a pin mill.

A loose powder with a bulk density of 13 g / l and a DBP number of was obtained 3.2 ml / g.

The matting effect of the substance was in the

Poly-urethane coating tested and can be found in Example 9.

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Example 8

The matting effect of the products obtained according to Examples 5 and 7 was black Baked enamel with that of a commercially available airgel produced according to the Kistler process compared. The degree of gloss according to Lange at a reflection angle of 45 ° and the Grindometer value according to Hegemann.

The following values were determined: ι ο

product

Grindometer value
in μπι

% Residual gloss
45 °

Commercial airgel 62
Product according to example 5 38
Product according to example 7 30

4.0 0.2 3.7

The paint used had the following composition:

7 parts by weight Carbon black paste (containing 18% carbon black,

38% alkyd resin and 44% white spirit
53 parts by weight fatty acid modified alkyd resin

75% in xylene
12 parts by weight Melamine resin 55% in butanol

4 parts by weight Butano!

2 parts by weight Ethyl glycol
16 parts by weight Xylene

2 parts by weight Glycolic acid butyl ester

2 parts by weight Butyl acetate 85%

2 parts by weight Silicone oil O% in xylene

5 parts by weight were incorporated in each case. Product. The incorporation was carried out by stirring for 10 minutes with a paddle stirrer at 2000 rpm. The paint was sprayed in about 30 μπι strong dry layer on sheets, air dried and baked for 30 min at 180 ⁰ C.

Example 9

The matting effect of the products obtained from Examples 1-7 was compared with that of common matting agents Poly-urethane coating that is very difficult to mattify, preferably matting with aerogels is checked. Amazingly, as the following table and graph show, one Relation between porosity, expressed by the DBP number, and the achieved matt level proven will. The matt level was measured according to Lange at a reflection angle of 60 ° (A b b. 3).

15th

20th

25th

30th

35

50

product

% Residual gloss
60 °

DBP number
(ml / g)

Output material 15.0 2.78 example 1 4.3 2.60 from example 1 5.0 3.10 from example 2 2.8 3.60 from example 3 0.7 3.78 from example 4 0.5 3.20 from example 5 1.6 3.20 from example 6 1.4 3.30 from example 7 1.5 Commercial product

The coating compound used had the following composition:

Polyurethane paint formulation

3.01 parts by weight Carbon black paste with vinyl acetate-vinyl chloride copolymers 0.62 parts by weight. blue pigment preparation in

Vinyl acetate-vinyl chloride copolymers 84.0 parts by weight. 30% polyester urethane in

Ethyl acetate
10-20 parts by weight Ethyl acetate

3.62 parts by weight were added to this mass. product to be tested incorporated with the spatula and then Dispersed for 4 min with a dissolver at 2000 rpm.

Thereafter, 4.22 parts by weight were in each case. Imprafix® and Desmodur® L (both commercial products are polyfunctional, aromatic isocyanates) stirred in with a spatula and with a 300 μm drawing iron on cardboard raised.

After the spread had dried in the air, it was ^{hardened for 30 min at 80 °} C. in a drying cabinet.

Example 10

In a Lödige mixer, 400 g of a fumed silica with a bulk density of 25 g / l and a BET surface area of 298 m ² / g are mixed with 100 ml of a 2.5% silica sol with a pH of 6.3, moisturizes.

The humidification takes place with mixing stirring by adding the humidifying agent dropwise within 15 minutes. Mixing is then carried out for 30 minutes.

A dry-acting powder with a water content of 20% is obtained. This powder is milled in an air jet mill with a grinding pressure of 6 bar at a throughput of 12.5 kg / h and dried in a cylinder at 11O ⁰ C in a drying cabinet.

A loose powder with a bulk density of 11 g / l and a specific surface area of 264 m ² / g is obtained.

The Hegemann grindometer value is determined to be 45 micrometers, the degree of gloss combined The polyurethane coating mass is 1.3% (determined as indicated in Example 9).

Example 11

In a "Lödige mixer 400 g of pyrogenic silica with a bulk density of 25 g / l and a BET surface area of 298 m ² / g with 100 ml of a dilute aqueous solution of sodium methylsiliko with an SiO ₂ content of 1.8 percent by weight has, moistened with mixing stirring within 15 minutes. The mixture is then mixed for 30 minutes.

A dry-acting powder with a water content of 20% is obtained, which is ground in an air jet mill with a grinding pressure of 6 bar and a throughput of 11 kg / h. The drying takes place in a cylinder at 110 ⁰ C in the drying cabinet,

The result is a loose powder with a bulk weight of 12 g / l and a specific surface area of 216 m ² / g. The Hegeman grindometer value is 37 micrometers, the degree of gloss in a polyurethane coating compound is 1.8% (determined as indicated in Example 9).

For this purpose 3 sheets of drawings 709 547/3

m ra ^ g -s

Claims (10)

Patent claims: 1. Airgel-like structured silica with a BET surface between 80 and 45Om ² Zg, a bulk density between 10 and 60, a DBP number between 2.4 and 3.8, an outer particle size range from 0.1 to 7 μm, a most common particle size range from 1 to 2 μίτι and a pH in 4% aqueous slurry of 6 to 8, obtained by incorporating 5 to 50 wt .-% to a pH between 7 and 14 adjusted water in air-dispersed fumed silica with a bulk weight from 10 to 60 g / i and a BET surface area between 100 and 480 m ² / g with uniform distribution and drying of the powdery mixture obtained. 2. A process for the production of airgel-like structured silica, characterized in that that in air-dispersed pyrogenic silica with a bulk density of 10-60 g / l and a BET surface area between 100 and 480nvVg 5-50% by weight to a pH value between 7 and 14 adjusted water is incorporated with even distribution and the powdery mixture obtained dries. 3. The method according to claim 2, characterized in that one has silica with a bulk density from 15 to 30 starts. 4. The method according to claims 2 and 3, characterized in that one in the water Adjusts pH value from 10 to 11. 5. Process according to claims 2 to 4, characterized in that the water-free silica or hydrolytically adding silicic acid and / or alkali-releasing substances. 6. Process according to claims 2 to 5, characterized in that the water-laden Silica before drying in a closed vessel for 5 to 60 minutes at temperatures up to Steams the boiling point of the water. 7. The method according to claims 2 to 6, characterized in that the water-laden Milled silica to further improve water distribution. 8. The method according to claims 2 to 6, characterized in that the water-laden Silica is ground and dried at the same time in a steam or air jet mill. 9. Process according to claims 2 to 7, characterized in that there is dry grinding on a pin or air jet mill. 10. Use of the silica of claim 1 obtained according to claims 2 to 9 as a matting agent. ok